The present invention relates to a gradient index optical element applicable to an optical lens such as a camera, a microscope or the like.
Such a gradient index optical element has a power (refractive power) in its medium by imparting a distribution of a refractive index to the medium. This power is determined by the refractive index distribution, so that in order to make the power large, the absolute value .vertline..DELTA.n.vertline. of a difference .DELTA.n in the refractive index n must be made large. Heretofore, many attempts of making the absolute value .vertline..DELTA.n.vertline. large have been done by many searchers. For example, an optical element which is commercially available under the name of SELFOC lens (Trade name) has a large absolute value .vertline..DELTA.n.vertline. by imparting a concentration gradient of Tl to the medium by ion exchange. Also, a lens of .vertline..DELTA.n.vertline..apprxeq.0.059 was obtained by imparting a concentration gradient of Ag to the medium by a double ion exchange method (Applied Optics, vol. 27, no. 3 (1988) p.496). Moreover, a lens of .vertline..DELTA.n.vertline..apprxeq.0.04 was obtained by imparting a concentration gradient of Pb and K to the medium by the sol-gel method (J.Non-cry.sol.100, 506, 1988). Finally, a lens of .vertline..DELTA.n.vertline..apprxeq.0.03 was obtained by imparting a concentration gradient of Ti or Ge to the medium by the sol-gel method (Elect.Lett.22, 99 (1986), Elect.Lett.22, 1108 (1986)).
However, the developments in the gradient index optical element up to the present have mainly involved an approach resulting in increasing the absolute value .vertline..DELTA.n.vertline., but measures to decrease the chromatic aberration possessed by the optical elements themselves have lagged behind. In designing optical elements, it is possible to drastically decrease the number of lenses constructing the optical system for a lens system of a camera by using the excellent aberration correction ability of the gradient index optical element, but there is an inconsistency that the chromatic aberration correction of the lens system becomes more difficult as the number of the lenses decreases. In order make a lens system in which the number of lenses is small and the chromatic aberration is corrected, it is important to decrease the chromatic aberration itself generated in each lens. Therefore, the following properties are desired for the medium of a gradient refractive index optical element.
In a radial gradient refractive index optical element, the refractive index of the medium differs depending on the position through which a light beam is passing (the distance from the axis), and also the refractive index of the light beam differs. The Abbe number .upsilon..sub.d of the medium may be designated by .upsilon..sub.d =(n.sub.d -1)/(n.sub.F -n.sub.C). If this Abbe number .upsilon..sub.d is constant from the above equation, the dispersion (n.sub.F -n.sub.C) increases as the refractive index n.sub.d increases. Therefore, in order to prevent the chromatic aberration from increasing for an increase of refractive index, it is necessary to increase the Abbe number .upsilon..sub.d as the refractive index n.sub.d is increased (refer to Japanese Patent Laid-Open No. 141,302/91). That is, it is desirable to obtain an optical element having a gradient of concentration distribution in the direction A on the n.sub.d -.upsilon..sub.d graph shown in FIG. 5.
The gradient index optical element is then obtained by imparting a gradient of refractive index to the medium on the basis of a gradient of concentration of metal oxide in a glass. The optical property of the glass is determined by its oxide composition. For glass having as its principal component SiO.sub.2, metal oxides except for Si have a high refractive index as compared with SiO.sub.2 glass, and dispersion becomes large (that is, the Abbe number becomes small), so that if a gradient index optical element is obtained by imparting a concentration gradient to these metal dopants, only the gradient index optical element having a gradient of concentration distribution in the direction B on the n.sub.d -.upsilon..sub.d graph shown in FIG. 5 is obtained.
Japanese Patent Laid-Open No. 141,302/91 discloses a gradient index optical element obtained by imparting concentration distributions in opposite directions to each other to the medium with the use of two or more metals, in which the concentration distribution property may be variously changed. However, it is difficult to obtain the gradient index optical element, since the above method of imparting concentration distributions of two metals in opposite directions to each other is limited. That is, heretofore, it is difficult to obtain the optical element having optical properties in the direction A on the n.sub.d -.upsilon..sub.d graph shown in FIG. 5 and desired in the designing of optical devices at the point of material production.